1. Field
The present disclosure relates to redox flow batteries, and more particularly, to redox flow batteries showing high energy densities.
2. Description of the Related Art
Generally, a secondary battery stores energy by transforming electrical energy into chemical energy through a charging process and uses energy by transforming the chemical energy into electrical energy.
Likewise, a redox flow battery (hereinafter, “RFB”) uses energy through a charging and discharging process by transforming electrical energy into chemical energy or by transforming chemical energy into electrical energy. However, a unique aspect of the redox flow battery is that unlike a general secondary battery, an electrode active material that stores energy exists in a liquid form instead of a solid form. Accordingly, in a redox flow battery, the electrode active material is stored in a tank, and voltage of the battery may be increased through stacking unit cells.
In greater detail, a catholyte and an anolyte act as electrode active materials, and typically, a transition metal oxide exists in a liquid state, wherein the transition metal oxide is dissolved in a solvent. Hence, the catholyte and the anolyte are stored in the tank as a mixture of reduced and oxidized electrolytes.
Similar to a fuel cell, a cell generating electrical energy includes a structure of carbon electrode/membrane/carbon electrode. In such a cell, the catholyte and the anolyte supplied by pumps undergo an oxidation reduction reaction at the surfaces of each carbon electrode, generating an electromotive force corresponding to Gibbs free energy. The carbon electrode does not directly contribute to the reaction and only supports the oxidation/reduction reaction of active materials. The membrane does not directly contribute to the reaction; however, it performs a function of rapidly transporting charge carrier ions between the catholyte and the anolyte, thus preventing a positive electrode and a negative electrode from directly contacting each other, and most importantly, suppressing a crossover of active ions dissolved in the catholyte and the anolyte.
However, a conventional redox flow battery using an aqueous solvent has a disadvantage of low energy density caused by a low driving voltage as an operation potential is limited to the water decomposition potential. Accordingly, there is a continuous demand for a redox flow battery having a high energy density.